Prior art processes exist for the removal of sulfur dioxide (SO.sub.2) from polluted gas streams. These processes yield a concentrated SO.sub.2 off-gas. Such a process is described in Steiner et al, Removal and Reduction of Sulfur Dioxides from Polluted Gas Streams, 15 American Chemical Society's Advances in Chemistry Series, No. 139, at 180 (1975). The present invention relates to the removal of SO.sub.2 from an SO.sub.2 -containing off-gas, and its conversion to elemental sulfur.
In U.S. Pat. No. 4,147,762 (hereafter the "762 patent"), a process is described for removing SO.sub.2 from SO.sub.2 -containing off-gas and converting it to elemental sulfur. More specifically, steam is introduced into SO.sub.2 -containing off-gas and the composite is passed through a reactor containing coal. The process described in the 762 patent has two significant advantages: (1) relatively inexpensive high sulfur coal can be used as the reducing agent; and (2) the conversion reaction can be conducted at temperatures that are substantially lower than those required in previously known processes. Notwithstanding these advantages, the process of the 762 patent has two significant problems: (1) the utilization of carbon, the reducing agent, is low; and (2) the yield of elemental sulfur, at high percentages of SO.sub.2 removal, is low, with the concomitant production of undesirable by-products such as H.sub.2 S, COS, and CS.sub.2.